Velocities of acoustic compressional (P) and shear (S) waves were measured to 8 GPa at room temperature for dense (99.7% of theoretical density), isotropic polycrystalline specimens of MgSiO3-majorite garnet using the phase comparison method of ultrasonic interferometry in a 1000-ton split-cylinder apparatus (USCA-1000). The pressure derivatives of the adiabatic bulk modulus (KS) and the shear modulus (G) are K0'=(∂Ks/∂P)T=6.7¿0.4 and G0'=(∂G/∂P)T=1.9¿0.1. Substitution of Si for Mg and Al in these garnets increase K0' by 26%, and G0' by 16%, from Mg3Al2Si3O12-pyrope to majorite. The high values of K0' and G0' from this study lead to much steeper velocity-depth profiles in the transition zone (400--700 km) than those calculated from the K0' and G0' estimates from elasticity systematics by Duffy and Anderson (1989). ¿ 1998 American Geophysical Union